docs: update all docs for /model command overhaul and custom provider support

Documents the full /model command overhaul across 6 files:

AGENTS.md:
- Add model_switch.py to project structure tree

configuration.md:
- Rewrite General Setup with 3 config methods (interactive, config.yaml, env vars)
- Add new 'Switching Models with /model' section documenting all syntax variants
- Add 'Named Custom Providers' section with config.yaml examples and
  custom:name:model triple syntax

slash-commands.md:
- Update /model descriptions in both CLI and messaging tables with
  full syntax examples (provider:model, custom:model, custom:name:model,
  bare custom auto-detect)

cli-commands.md:
- Add /model slash command subsection under hermes model with syntax table
- Add custom endpoint config to hermes model use cases

faq.md:
- Add config.yaml example for offline/local model setup
- Note that provider: custom is a first-class provider
- Document /model custom auto-detect

provider-runtime.md:
- Add model_switch.py to implementation file list
- Update provider families to show Custom as first-class with named variants

website/docs/reference/cli-commands.md

@@ -98,8 +98,25 @@ Use this when you want to:
 - switch default providers
 - log into OAuth-backed providers during model selection
 - pick from provider-specific model lists
+- configure a custom/self-hosted endpoint
 - save the new default into config
 
+### `/model` slash command (mid-session)
+
+Switch models without leaving a session:
+
+```
+/model                              # Show current model and available options
+/model claude-sonnet-4              # Switch model (auto-detects provider)
+/model zai:glm-5                    # Switch provider and model
+/model custom:qwen-2.5              # Use model on your custom endpoint
+/model custom                       # Auto-detect model from custom endpoint
+/model custom:local:qwen-2.5        # Use a named custom provider
+/model openrouter:anthropic/claude-sonnet-4  # Switch back to cloud
+```
+
+Provider and base URL changes are persisted to `config.yaml` automatically. When switching away from a custom endpoint, the stale base URL is cleared to prevent it leaking into other providers.
+
 ## `hermes gateway`
 
 ```bash